The invention is directed to an overspeed safety means for fuel injection pumps of internal combustion engines having a valve closable upon interruption of an electrical circuit to shut off fuel. It is known to limit the rpm in Diesel engines by using a mechanical centrifugal governors in combination with injection pumps. If the control element of such a centrifugal governor exceeds a predetermined amount of deflection, the delivery of fuel by the fuel injection pump is thereby affected.
Especially with Diesel engines where the fuel injection quantity is regulated electronically, the problem of overspeed safety occurs. Either the already known mechanical overspeed safety means have to be used together with the electronic regulating units, or else specialized electronic overspeed safety means have to be created which function just as safely and reliably as the mechanical governors used heretofore.
In known overspeed safety means an rpm transducer is provided which emits an rpm-dependent voltage which, amplified by an amplifier circuit, controls a relay in the current circuit of the valve. Even the starting rpm of the engine is sufficient to obtain a volage at the output of the amplifier by means of which the relay is put into its closing position. Upon attaining a threshold rpm or a maximum permissible voltage at the input of the amplifier, a safety means in the input of the amplifier is actuated so that the amplifier is switched off and the relay is opened. This apparatus is quite expensive, however, and it includes sources of possible malfunction which can affect the reliability of the apparatus. Additional fail-safe provisions then have to be made, such as second shutoff safety means at the input of the amplifier.
An electrical centrifugal switch is also known, having a flyweight which deflects under the influence of centrifugal force; when the flyweight is in a specific deflected position, a switching contact can thereby be lifted, so that the supply of current to an electrically controlled fuel injection system is shut off. This apparatus has the disadvantage, however, that particularly when the contacts are not fully actuated at engine operating states near the threshold rpm range, the contacts can freeze or become soiled, thus no longer assuring the supply of current for an operating range which is in fact permissible. Even if the contact pairs are still functional, the current circuit can be closed again immediately after it has been interrupted, for instance because of the decreasing rpm caused by the failure of the fuel supply. Thus it becomes possible for the engine to be driven impermissibly in a harmful, high rpm range for a relatively long period. The continual switchovers occuring near this threshold range also damage the contacts as mentioned above. In particular, this type of safety means does not provide the operator of the engine with an opportunity to locate the source of the trouble causing the overspeeding.